1. Field of the Invention
The present invention relates to a cryostat provided with a refrigerator and more particularly to a vibration-insulated cryostat housing a superconducting magnet and provided with a refrigerator used in a nuclear magnetic resonance imaging apparatus.
2. Description of the Prior Art
A superconducting magnet (hereafter simply referred to as a magnet) is used for the purpose of obtaining a uniform high magnetic field and is mounted in a cryostat. Such magnets are particularly applicable in a nuclear magnetic resonance imaging instrument for physical diagnosis (hereafter simply referred to as an MRI) used in the medical field. In such apparatus, a vacuum-insulated cryostat which contains a cooled magnet is required. The magnet is cooled by immersing it in a refrigerant at a very low temperature such as liquefied helium in order to maintain the superconducting state. Since liquefied helium is evaporated gradually by the heat entering the cryostat, it is required to replenish the liquefied helium by filling periodically. If the rate of evaporation of the liquefied helium is reduced, the consumption of the liquefied helium is reduced and the operating cost is reduced by a large margin.
To reduce the helium evaporation rate, a refrigerator has been fitted to the cryostat, and the heat entering the vacuum container is absorbed by the refrigerator. Since this refrigerator is generally of an expander type using a reciprocating system, driving sound and impact sound of the piston are produced during operation, which are propagated to the vacuum container of the cryostat, thus generating considerable noise. Examples of support of the refrigerator on the cryostat tank itself are shown in U.S. Pat. No. 4,777,807 and U.S. Pat. No. 4,510,771.
To avoid transmitting noise and vibration to the cryostat tank, it is proposed in EP-A-359262 (corresponding to U.S. Pat. No. 4,959,964) to support the refrigerator on a magnetic shield around the tank. In this case, the refrigerator is supported by fixing it directly to one sheet of a group of magnetic shield sheet members disposed at predetermined intervals around the outer circumference of the cryostat, and the cryostat and the refrigerator are connected airtightly by means of bellows. However, the present inventors have found that, since the refrigerator was fixed at almost a central part of a planar magnetic shield member, the magnetic shield member was easily deformed in a bending direction of its plane due to excitation by vibration of the refrigerator. Therefore, there have been problems that the magnetic shield member itself is resonant with vibration of the refrigerator in a low frequency band. Due to the large vibration amplitude, noise is generated from the magnetic shield members, the vibration is transmitted to the cryostat which is coupled at the lower part of the magnetic shield, and noise is also generated from the cryostat.
If the plate thickness of the magnetic shield member is increased, thereby to make its vibration amplitude small and increase the natural frequency of the magnetic shield member so as to prevent a resonance phenomenon, there is the problem that the weight of the magnetic shield member is increased.